protein (in Synechocystis sp. PCC6803), or to exchange the constitutive D1:1 protein to an alternate D1:2 isoform (in Synechococcus sp.
PCC7942). To define more general patterns of cyanobacterial psbA expression, we applied moderately photoinhibitory UVB to Anabaena sp.
PCC7120 and tracked the expression of its five psbA genes. psbAI, encoding a D1:1 protein isoform characterized by a Gln130, represented the
majority of the psbA transcript pool under control conditions. psbAI transcripts decreased upon UVB treatment but the total psbA transcript pool
increased 3.5 fold within 90 min as a result of sharply increased psbAII, psbAIV and psbAIII transcripts encoding an alternate D1:2 protein isoform
characterized by Glu130, similar to that of Synechococcus. Upon UVB treatment the relaxation of flash induced chlorophyll fluorescence showed
a characteristic acceleration of a decay phase likely associated with the exchange from the D1:1 protein isoform encoded by psbAI to the alternate
D1:2 isoform encoded by psbAIV, psbAII and psbAIII. Throughout the UVB treatment the divergent psbA0 made only a trace contribution to the
total psbA transcript pool. This suggests a similarity to the divergent psbAI gene from Synechocystis, whose natural expression we demonstrate for
the first time at a trace level similar to psbA0 in Anabaena. These trace-expressed psbA genes in two different cyanobacteria raise questions
concerning the functions of these divergent genes.